The term symmetric in the context of resolution here used refers to the same dimension for each bit in both row and column direction (rows and columns being orthogonal in the traditional sense). Thus, a 600.times.600 dots per inch bit map is symmetric since the orthogonal dimensions for each bit are the same 1/600 inch. A bit map which is increased or compressed the same proportion in both the row and column dimensions is acted upon symmetrically.
Increasing and decreasing resolution symmetrically is a common technique to compress data. U.S. Pat. Nos. 4,280,143 to Judd and 5,020,115 to Black are illustrative. Such compression or expansion seeks to maintain a close replica of the bit map while conforming it in size to the capabilities of the imaging device with which it is to be used or simply to reduce data storage requirements.
This invention is directed to conforming bit map data available to the imaging device in one resolution to the resolution capabilities of the imaging device when those capabilities are not symmetric while the data is symmetric. Specifically, the preferred embodiment data for a printer is received in bit map form in 600.times.600 bits per inch resolution. The printer has the capability of printing in 300.times.1200 dots per inch, and the received bit map is modified to use the resolution of the printer to provide excellent printing.
The printer which has the 300.times.1200 dots per inch capability employs a commercially available light emitting diode (LED) printhead. The full 300.times.1200 capability of such a printhead has been used in the prior art for gray scale printing and for printing which eliminates the stair step effect of bit map printing (termed enhancement of resolution). Enhancement of resolution involves the observation of a, for example, 300.times.300 data bit map for patterns characteristic of the stair step and the modification of the printing using the 1200 capability to smooth the stair steps. This invention creates a new data bit map without regard to characteristic pattern but simply by rearranging all of the bits in an original bit map in a manner not varied by the content of the bit map.